How to achieve inter-instance synchronization in java?

Question

Suppose I have a Java Spring application having a synchronized method (thread-safe). If my application is deployed on a single instance, then I can say that it's thread safe. However, to scale, we must deploy on multiple instances. How to achieve inter-instance synchronization?

Answer 1

The question doesn't seem to be about thread-safety. A single application can have multiple threads, and certain methods in an application can be thread-safe or -unsafe.

With that, if you have multiple instances of a given application that needs access to communicate, then its as @Michael in the comments said: it is a form of inter-process communication (IPC). There are two ways to achieve this: message passing and shared memory. Some examples of this can be found at this answer: How to have 2 JVMs talk to one another

However, since you are using Java Spring, a web framework, you are likely already working with network based services. Why not have multiple instances of the same service share state through an external shared service? This sounds like a great use-case for Message Brokers like Redis or RabbitMQ. As Reddis states as their homepage states:

Chat, messaging, and queues

Redis supports Pub/Sub with pattern matching and a variety of data structures such as lists, sorted sets, and hashes. This allows Redis to support high performance chat rooms, real-time comment streams, social media feeds and server intercommunication. The Redis List data structure makes it easy to implement a lightweight queue. Lists offer atomic operations as well as blocking capabilities, making them suitable for a variety of applications that require a reliable message broker or a circular list.